First, the band gap substantially decreases towards the Sn-end (\(x=1\) ). As computed with the HSE06 hybrid functional [1]}, [2]} with SOC, the band gap is 0.53 eV for Mg\(_{2}\) Si, 0.19 eV for Mg\(_{2}\) Si\(_{0.25}\) Sn\(_{0.25}\)  (which is already fairly small) where the bands are close to converging, and 0.02 eV for Mg\(_{2}\) Sn (essentially semimetallic). Therefore, increasing bipolar effect is likely a big reason that the PF falls toward the Sn-end as \(x\)  increases beyond some optimal value.
